For gas turbine engines, it is generally known that the operational clearances between static engine structures and the tips of rotating blades impact the thermodynamic efficiency and fuel burn (e.g., specific fuel consumption or SFC) of the engine. Hence, gas turbine engine manufacturers continually seek ways to reduce these operational clearances, while at the same time avoiding rubs between the rotating blade tips and the static structure. The value of even several thousandths of an inch improvement can be quite significant.
Unfortunately, the lengths of the blade tips typically vary at a different rate than the static structures can expand or contract to accommodate the change in blade tip length, especially during transient operations. This can result in the blade tips contacting the static structure or cause excess clearance between the blade tips and static structure, both of which can reduce engine performance. One method that has been implemented to match the different growth rates is to supply a flow of air from the engine onto various rotor and/or static structures to reduce the operational clearances during steady state, high altitude cruise conditions.
Many gas turbine engines use either a static tip clearance control system or an active tip clearance control system. With typical static systems, cooling air is supplied through metered passages at a constant percentage of core flow rate. With typical active systems, complex tubes, manifolds and/or extra layers of turbine cases are used, which can significantly increase weight and cost, and often use air from different engine stations, which can significantly increase complexity and potential failure modes.
Hence, there is a need for a relatively simple tip clearance control system that fits within a current gas turbine engine envelope and that uses local compressor discharge air to feed directly into the turbine case system. The present invention addresses at least this need.